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}</style><title>Node Web学习笔记（3）</title></head><body><article class="markdown-body">
<h2>
<a id="user-content-tags-事件驱动" class="anchor" href="#tags-%E4%BA%8B%E4%BB%B6%E9%A9%B1%E5%8A%A8" aria-hidden="true"><span class="octicon octicon-link"></span></a>Node Web学习笔记（3）-事件驱动，监听与触发</h2>

<p>在学习过程中，渐渐的会引出关于Node的核心概念，在进一步的学习中，我们就需要弄清楚这些核心概念的意思，这里有必要写到，Node是什么？（我习惯直接叫Node，等同于Node.js）</p>

<blockquote>
<p>Node.Js是基于javascript语言，建构在google V8 engine以及Linux上的一个单进程单线程、非阻塞事件驱动IO框架。</p>
</blockquote>

<p>上篇文章通过<code>fs</code>模块的两个方法简要介绍了什么是异步，这次简要介绍一下什么是事件驱动机制，可以这么说，正是由于有事件驱动机制，才能使Node成为非阻塞IO框架。想继续了解，可以看一下这篇文章<a href="http://www.infoq.com/cn/articles/tyq-nodejs-event/">深入浅出Node.js（四）：Node.js的事件机制</a>。</p>

<blockquote>
<p>Node.js中大部分的模块，都继承自<a href="http://nodejs.org/docs/latest/api/events.html">Event模块</a>。Event模块（events.EventEmitter）是一个简单的事件监听器模式的实现类。它的对象具有<code>addListener</code>，<code>on</code>，<code>once</code>，<code>removeListener</code>，<code>removeAllListeners</code>，<code>emit</code>等基本的事件监听模式的方法实现。</p>
</blockquote>



<p>下面看个简单的事件监听和触发的例子。</p>

<div class="highlight highlight-javascript"><pre><span class="pl-s">var</span> events <span class="pl-k">=</span> <span class="pl-s3">require</span>(<span class="pl-s1"><span class="pl-pds">"</span>events<span class="pl-pds">"</span></span>);
<span class="pl-s">var</span> emitter <span class="pl-k">=</span>  <span class="pl-k">new</span> <span class="pl-en">events.EventEmitter</span>();  <span class="pl-c">//创建了事件监听器的一个对象</span>

<span class="pl-c">// 监听事件some_event</span>
emitter.on(<span class="pl-s1"><span class="pl-pds">"</span>some_event<span class="pl-pds">"</span></span>, <span class="pl-st">function</span>(){
    <span class="pl-en">console</span><span class="pl-s3">.log</span>(<span class="pl-s1"><span class="pl-pds">"</span>事件触发，调用此回调函数<span class="pl-pds">"</span></span>);
});

<span class="pl-s3">setTimeout</span>(<span class="pl-st">function</span>(){
    emitter.emit(<span class="pl-s1"><span class="pl-pds">"</span>some_event<span class="pl-pds">"</span></span>);     <span class="pl-c">//触发事件some_event</span>
},<span class="pl-c1">3000</span>); </pre></div>

<p>代码做了一个简单的注释，应该很容易理解了，在这里我们是监听了一个事件，然后通过定时器3秒后触发了该事件，触发事件后，程序立即调用回调函数，打印出文本到终端上。这就是这个程序的逻辑。这个程序主要应用了<code>on</code>、<code>emit</code>方法，一个是监听，一个是触发。</p>

<blockquote>
<p>Node里面的许多对象都会分发事件：一个<code>net.Server</code>对象会在每次有新连接时分发一个事件， 一个<code>fs.readStream</code>对象会在文件被打开的时候发出一个事件。 所有这些产生事件的对象都是<code>events.EventEmitter</code>的实例。</p>
</blockquote>

<p>既然这样，那么回过头来看看，咱们第一次写的那个最简单的Web服务器代码。</p>

<div class="highlight highlight-javascript"><pre><span class="pl-s">var</span> http <span class="pl-k">=</span>  <span class="pl-s3">require</span>(<span class="pl-s1"><span class="pl-pds">"</span>http<span class="pl-pds">"</span></span>);
<span class="pl-s">var</span> <span class="pl-en">onRequest</span> <span class="pl-k">=</span> <span class="pl-st">function</span>(<span class="pl-vpf">req</span>, <span class="pl-vpf">res</span>){
    res.writeHead(<span class="pl-c1">200</span>,{<span class="pl-s1"><span class="pl-pds">"</span>Content-Type<span class="pl-pds">"</span></span><span class="pl-k">:</span> <span class="pl-s1"><span class="pl-pds">"</span>text/html<span class="pl-pds">"</span></span>});
    res.end(<span class="pl-s1"><span class="pl-pds">"</span>Hello, Node.js<span class="pl-pds">"</span></span>);
}
<span class="pl-s">var</span> server <span class="pl-k">=</span> http.createServer(onRequest);
server.listen(<span class="pl-c1">8888</span>);
<span class="pl-en">console</span><span class="pl-s3">.log</span>(<span class="pl-s1"><span class="pl-pds">"</span>Server is started @http://localhost:8888<span class="pl-pds">"</span></span>);</pre></div>

<p>在上面的代码中，同样是事件触发，调用回调函数，但是在这段代码中，我们并不能很直观的看到，什么时候触发了事件，又是什么时候调用了回调函数，为什么呢？<em>因为这些代码都被封装在<code>createServer()</code>方法里了</em>，接下来看下一段代码，同样是一个简单的Web服务器，效果和上面的代码一样。</p>

<div class="highlight highlight-javascript"><pre><span class="pl-s">var</span> http <span class="pl-k">=</span> <span class="pl-s3">require</span>(<span class="pl-s1"><span class="pl-pds">"</span>http<span class="pl-pds">"</span></span>);
<span class="pl-s">var</span> server <span class="pl-k">=</span> <span class="pl-k">new</span> <span class="pl-en">http.Server</span>();  <span class="pl-c">//创建一个服务器类的对象</span>
server.on(<span class="pl-s1"><span class="pl-pds">"</span>request<span class="pl-pds">"</span></span>, <span class="pl-st">function</span>(<span class="pl-vpf">req</span>, <span class="pl-vpf">res</span>){
    res.writeHead(<span class="pl-c1">200</span>, {<span class="pl-s1"><span class="pl-pds">"</span>Content-Type<span class="pl-pds">"</span></span><span class="pl-k">:</span> <span class="pl-s1"><span class="pl-pds">"</span>text/html<span class="pl-pds">"</span></span>});
    res.end(<span class="pl-s1"><span class="pl-pds">"</span>Http服务器-手动监听<span class="pl-pds">"</span></span>);
});
server.listen(<span class="pl-c1">8888</span>);
<span class="pl-en">console</span><span class="pl-s3">.log</span>(<span class="pl-s1"><span class="pl-pds">"</span>Server is started @http://localhost:8888<span class="pl-pds">"</span></span>);</pre></div>

<p>这里没有用<code>createServer()</code>方法，而是手动创建了一个<code>http.Server</code>对象监听request事件，当服务器端收到浏览器访问请求时，就会立即触发<code>request</code>事件，调用回调函数，返回页面。两段代码的逻辑一模一样，多琢磨一下，对理解事件机制会有一定的帮助的。</p>

<p>关于Node的事件驱动就先写到这里，下面附上如何创建多个的事件监听器的代码，可以对比学习一下。</p>

<div class="highlight highlight-javascript"><pre><span class="pl-s">var</span> events <span class="pl-k">=</span> <span class="pl-s3">require</span>(<span class="pl-s1"><span class="pl-pds">'</span>events<span class="pl-pds">'</span></span>);
<span class="pl-s">var</span> emitter <span class="pl-k">=</span> <span class="pl-k">new</span> <span class="pl-en">events.EventEmitter</span>();
<span class="pl-c">/**</span>
<span class="pl-c">*   设置两个监听器</span>
<span class="pl-c">*/</span>
emitter.on(<span class="pl-s1"><span class="pl-pds">'</span>someEvent1<span class="pl-pds">'</span></span> , <span class="pl-st">function</span> (<span class="pl-vpf">argv1</span>, <span class="pl-vpf">argv2</span>) {
    <span class="pl-en">console</span><span class="pl-s3">.log</span>(<span class="pl-s1"><span class="pl-pds">'</span>listen1：<span class="pl-pds">'</span></span> <span class="pl-k">+</span> argv1 <span class="pl-k">+</span> argv2);
});
emitter.on(<span class="pl-s1"><span class="pl-pds">'</span>someEvent2<span class="pl-pds">'</span></span>, <span class="pl-st">function</span> (<span class="pl-vpf">argv1</span>, <span class="pl-vpf">argv2</span>) {
    <span class="pl-en">console</span><span class="pl-s3">.log</span>(<span class="pl-s1"><span class="pl-pds">'</span>listen2：<span class="pl-pds">'</span></span> <span class="pl-k">+</span> argv1 <span class="pl-k">+</span> argv2);
});
<span class="pl-c">/*</span>
<span class="pl-c">*   触发事件</span>
<span class="pl-c">*/</span>
emitter.emit(<span class="pl-s1"><span class="pl-pds">"</span>someEvent1<span class="pl-pds">"</span></span>,<span class="pl-s1"><span class="pl-pds">'</span>Hello，<span class="pl-pds">'</span></span>,<span class="pl-s1"><span class="pl-pds">'</span>Node<span class="pl-pds">'</span></span>);

<span class="pl-s3">setTimeout</span>(<span class="pl-st">function</span>(){
    emitter.emit(<span class="pl-s1"><span class="pl-pds">"</span>someEvent2<span class="pl-pds">"</span></span>,<span class="pl-s1"><span class="pl-pds">'</span>Hello，<span class="pl-pds">'</span></span>,<span class="pl-s1"><span class="pl-pds">"</span>2015<span class="pl-pds">"</span></span>);
},<span class="pl-c1">3000</span>);</pre></div>
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